This invention relates to imaging devices, particularly but not exclusively so-called hybrid devices which may comprise infrared detector elements of, for example, cadmium mercury telluride coupled to capacitor means in, for example, a silicon circuit body. Such devices may be used in so-called "solid-state" video cameras and may operate at infrared wavelengths, for example in 3 to 5 .mu.m (micrometers) or 8 to 14 .mu.m bands.
In European patent application (EP-A) No. 0 128 828, there is disclosed an imaging device comprising (a) a plurality of photovoltaic detector elements which generate photocurrent in response to incident photon radiation, (b) capacitor means coupled to each detector element for integrating the current generated at that detector element during an integration period, the capacitor means being at least partially discharged by said current from the detector element, (c) an injection gate for controlling injection of the current from the detector element to the capacitor means, and (d) reset means for resetting the potential of the capacitor means to charge the capacitor means before the beginning of the integration period.
Such imaging devices disclosed in EP-A No. 0 128 828 differ from previous imaging devices using charge-coupled devices (CCDs) for processing and reading the signals from the detector elements. In particular by integrating the detector element current in such resettable capacitor means and by reading the capacitor potential using voltage-reading means switchably coupled to the capacitor means, it is possible to avoid certain disadvantages of charge-coupled devices in requiring high operating voltages, in introducing charge-transfer noise between successive stages of the CCD and in limiting the signal handling capability to values which are sometimes significantly less than what is desirable particularly for infrared imaging. Furthermore, the technology for fabricating CCDs is more complex than MOS transistor technology (for example, n channel MOST technology) with which the signal processing circuit of this type of imaging device can be fabricated.
The devices disclosed in EP-A No. 0 128 828 do not include any so-called "anti-blooming" or "blooming-protection" means. Such protection means are frequently necessary in charge-coupled devices to prevent excessive charge (generated by, for example, a very bright area of the scene being imaged) at one charge-storage site flowing over into neighbouring storage sites of the charge-coupled device so corrupting the signals represented by the charge at these neighbouring sites. Such charge overflow does not occur between the capacitors and transistors of the imaging devices disclosed in EP-A No. 0 128 828, due to the different mode of integrating and reading the photocurrent signal output from the detector elements in the devices of EP-A No. 0 128 828. For particular examples of blooming-protection means already known for charge-coupled devices reference is invited to U.S. Pat. No. 3,896,474 and to United Kingdom patent GB-A No. 1,413,092.
However it has been noted by the present inventor that even in the imaging devices of EP-A No. 0 128 828 excessive current generated at a detector element can still present difficulties. A capacitor means may not only become fully discharged by excessive charge generated at a detector element, but the potential of the capacitor means may then start to invert. This excessive photocurrent can change the bias condition for the detector elements. Thus, for example, infrared imaging devices for 3 to 5 .mu.m or 8 to 14 .mu.m wavelength operation may be based most conveniently on photovoltaic infrared detector elements operated in zero-bias condition and formed in a body of suitable infrared sensitive material, for example cadmium mercury telluride. Excessive photocurrent at one such detector element can not only invert the capacitor potential but also forward bias the photovoltaic element which may then inject charge carriers into a common region of the infrared sensitive body where they interfere with the other detector elements and the charge generated by the incident radiation at these other detector elements.